Life support systems on the ISS provide oxygen, absorb
carbon dioxide, and manage vaporous emissions from the astronauts
themselves. It's all part of breathing easy in our new home in
space.

November 13, 2000 -- Many of us
stuck on Earth wish we could join (at least temporarily) the
Expedition
1 crew aboard the International
Space Station (ISS). Floating effortlessly from module to
module, looking down on Earth from a breathtaking height of 350
kilometers.... It's a dream come true for innumerable space lovers.

Right: An artist's rendering
of the ISS as it currently appears.

But be careful what you wish for! Living on the Space Station
also means hard work, cramped quarters, and... what's that smell?
Probably more outgassing from a scientific experiment or, worse
yet, a crewmate.

With 3 to 7 people sharing a small enclosed volume on the
still-growing Space Station, air management is critical.

Life support systems on the ISS must not only supply oxygen
and remove carbon dioxide from the cabin's atmosphere, but also
prevent gases like ammonia and acetone, which people emit in
small quantities, from accumulating. Vaporous chemicals from
science experiments are a potential hazard, too, if they combine
in unforeseen ways with other elements in the air supply.

So, while air in space is undeniably rare, managing it is
no small problem for ISS life support engineers.

In this second article in a series about the practical challenges
of living in space, Science@NASA examines how the ISS
will provide its residents with the breath of life.

Making oxygen from water

Most people can survive only a couple of minutes without oxygen,
and low concentrations of oxygen can cause fatigue and blackouts.

To ensure the safety of the crew, the ISS will have redundant
supplies of that essential gas.

"The
primary source of oxygen will be water electrolysis, followed
by O2 in a pressurized storage tank," said Jay
Perry, an aerospace engineer at NASA's Marshall Space Flight
Center working on the Environmental Control and
Life Support Systems (ECLSS) project. ECLSS
engineers at Marshall, at the Johnson Space Center and elsewhere
are developing, improving and testing primary life support systems
for the ISS.

Most of the station's oxygen will come from a process called
"electrolysis," which uses electricity from the ISS
solar panels to split water into hydrogen gas and oxygen gas.

Left: The ISS's first crew
-- Bill Shepherd, Sergei Krikalev and Yuri Gidzenko -- aboard
the Space Station. During their four-month stay, the crew will
rely on the Station's hardware to provide breathable air.

Each molecule of water contains two hydrogen atoms and one
oxygen atom. Running a current through water causes these atoms
to separate and recombine as gaseous hydrogen (H2)
and oxygen (O2).

The oxygen that people breathe on Earth also comes from the
splitting of water, but it's not a mechanical process. Plants,
algae, cyanobacteria and phytoplankton all split water molecules
as part of photosynthesis -- the process that converts sunlight,
carbon dioxide and water into sugars for food. The hydrogen is
used for making sugars, and the oxygen is released into the atmosphere.

"Eventually,
it would be great if we could use plants to (produce oxygen)
for us," said Monsi Roman, chief microbiologist for the
ECLSS project at MSFC. "The byproduct of plants doing this
for us is food."

However, "the chemical-mechanical systems are much more
compact, less labor intensive, and more reliable than a plant-based
system," Perry noted. "A plant-based life support system
design is presently at the basic research and demonstration stage
of maturity and there are a myriad of challenges that must be
overcome to make it viable."

Hydrogen that's leftover from splitting water will be vented
into space, at least at first. NASA engineers have left room
in the ECLSS hardware racks for a machine that combines the hydrogen
with excess carbon dioxide from the air in a chemical reaction
that produces water and methane. The water would help replace
the water used to make oxygen, and the methane would be vented
to space.

Right: The oxygen that humans
and animals breathe on Earth is produced by plants and other
photosynthetic organisms such as algae.

"We're looking to close the loop completely, where everything
will be (re)used," Roman said. Various uses for the methane
are being considered, including expelling it to help provide
the thrust necessary to maintain the Space Station's orbit.

At present, "all of the venting that goes overboard is
designed to be non-propulsive," Perry said.

The ISS will also have large tanks of compressed oxygen mounted
on the outside of the airlock module. These tanks will be the
primary supply of oxygen for the U.S. segment of the ISS until
the main life support systems arrive with Node 3
in 2005. After that, the tanks will serve as a backup oxygen
supply.

Last week, while the crew were waiting for activation
of a water electrolysis machine on the Zvezda Service Module,
they breathed oxygen from "perchlorate candles," which
produce O2 via chemical reactions inside a metal canister.

"You've got a metallic canister
with this material (perchlorate) packed inside it," Perry
explained. "They shove this canister into a reactor and
then pull an igniter pin. Once the reaction starts, it continues
to burn until it's all used." Each canister releases enough
oxygen for one person for one day.

"It's really the same technology
that's used in commercial aircraft," he continued. "When
the oxygen mask drops down, they say to yank on it, which actuates
the igniter pin. That's why you have to give it a tug to begin
the flow of oxygen."

Keeping the air "clean"

At present, carbon dioxide is removed from the air by a machine
on the Zvezda Service Module based on a material called "zeolite,"
which acts as a molecular sieve, according to Jim Knox, a carbon
dioxide control specialist at MSFC.

The removed CO2 will be vented to space. Engineers
are also thinking of ways to recycle the gas.

In addition to exhaled CO2, people also emit small
amounts of other gases. Methane and carbon dioxide are produced
in the intestines, and ammonia is created by the breakdown of
urea in sweat. People also emit acetone, methyl alcohol and carbon
monoxide -- which are byproducts of metabolism -- in their urine
and their breath.

Activated charcoal filters are the primary method for removing
these chemicals from the air.

Above: This diagram shows the flow of recyclable ("regenerative")
resources in the Space Station's Environmental Control and Life
Support System (ECLSS).

Maintaining a healthy atmosphere is made even more complex
by the dozens of chemicals that will be used in the science experiments
on board the ISS.

"In a 30 year period, there could be any number of different
types of experimental facilities on board that could have any
number of chemical reagents," Perry said.

Some of these chemicals are likely to be hazardous, particularly
if they're allowed to combine in unforeseen ways, Perry said.
Keeping these chemicals out of the air will be vital for the
crew's health.

When the Space Station was first being designed, NASA engineers
envisioned a centralized chemical-handling system that would
manage and contain all the chemicals used for experiments. But
such a system proved to be too complex.

"The ability for the Station to provide generic monitoring
capability to try to cover the broad spectrum of chemicals that
15 plus years of basic research will require -- obviously that's
not something that the Station itself can provide," Perry
said.

"It really made much greater sense
that each experimental facility on board the lab module would
provide its own containment of its (chemicals), essentially maintaining
responsibility for the chemicals from cradle to grave,"
Perry said.

Left: An illustration showing
the location of Node 3, where the ECLSS life support equipment
will be housed. Note that the Station components in the line
of sight to Node 3 are transparent in this image.

A safety review for each proposed experiment will determine
the level of containment that the rack-mounted experiment facilities
must provide. In the event of a release, the crew will seal off
the contaminated module and then follow procedures for cleanup,
if possible.

But careful planning and well-designed hardware should minimize
the risk of this scenario, enabling the crew of the Space Station
to breathe easy.